Method and apparatus for detecting workpiece surface defects and for fixing the location thereof using magnetic particles

ABSTRACT

A novel method and apparatus for detecting workpiece surface defects is disclosed. In accordance with the invention, the workpiece is magnetized and a ferritic powder is sprayed over the surface thereof. Due to stray flux lines of the magnetic field occurring at the surface defects, the ferritic powder accumulates at such defects and effectively traces the same, rendering the defect visible. The tracing of the defects is then fixed or preserved automatically so that the defects remain visible for subsequent corrective maintenance operations after the workpiece is demagnetized and the powder falls off. In one embodiment of the invention, the &#39;&#39;&#39;&#39;fixing&#39;&#39;&#39;&#39; of the tracing occurs in that a colored marking material is applied or sprayed over the surrounding surface about the traced defects as well as over the ferritic powder which effects the tracing itself. The workpiece is then demagnetized and the ferritic powder and the colored marking material over the powder is removed. The remainder of the surface, however, remains dyed by the colored marking material thus outlining the surface defects and fixing or preserving the visible tracing thereof. This technique is referred to herein as a so-called &#39;&#39;&#39;&#39;negative&#39;&#39;&#39;&#39; process. In an alternative embodiment of the invention, a &#39;&#39;&#39;&#39;positive&#39;&#39;&#39;&#39; process is employed wherein the ferritic powder which traces the surface defects is permanently adhered to the surface by means of the application of heat. Subsequently, the workpiece is demagnetized leaving the permanently adhered ferritic powder at the defect location thus fixing the visible tracing.

United States Patent [72] Inventor Kurt Reinshagen Ottweiler (Saar), Germany [21] Appl. No. 873,043 [22] Filed Oct.3l, I969 [45] Patented Oct.19,l97l [73] Assignee Elektrodenfabrik Oerlikon Buhrle AG Zurich, Switzerland [32] Priority Nov. 8, 1968 [3 3] Switzerland [31 16709/68 [54] METHOD AND APPARATUS FOR DETECTING WORKPIECE SURFACE DEFECTS AND FOR FIXING THE LOCATION THEREOF USING MAGNETIC PARTICLES 8 Claims, 4 Drawing Figs. [52] U.S. Cl 324/38 [51] Int. Cl ..G0lr33/12 [50] Field of Search 324/38, 37; 250/71 T 56] References Cited UNITED STATES PATENTS 3,073,212 H1963 Dunsheath et al. 324/38 3,249,861 5/1966 Pevar 324/38 3,480,855 ll/l969 Lorenzi 324/38 FORElGN PATENTS l ,l69,696 5/1964 Germany 250/71 T Primary Examiner-Alfred E. Smith Assisian! E.\'uminerR. J. Corcoran Alturneywerner W. Kleeman ABSTRACT: A novel method and apparatus for detecting workpiece surface defects is disclosed. In accordance with the invention, the workpiece is magnetized and a ferritic powder is sprayed over the surface thereof. Due to stray flux lines of the magnetic field occurring at the surface defects, the ferritic powder accumulates at such defects and effectively traces the same, rendering the defect visible. The tracing ofthe defects is then fixed or preserved automatically so that the defects remain visible for subsequent corrective maintenance operations after the workpiece is demagnetized and the powder falls off. In one embodiment of the invention, the fixing" of the tracing occurs in that a colored marking material is applied or sprayed over the surrounding surface about the traced defects as well as over the ferritic powder which effects the tracing itself. The workpiece is then demagnetized and the ferritic powder and the colored marking material over the powder is removed. The remainder of the surface. however, remains dyed by the colored marking material thus outlining the surface defects and fixing or preserving the visible tracing thereof. This technique is referred to herein as a so-called negative" process. In an alternative embodiment of the invention, a positive" process is employed wherein the ferritic powder which traces the surface defects is permanently adhered to the surface by means of the application of heat. Subsequently, the workpiece is demagnetized leaving the permanently adhered ferritic powder at the defect location thus fixing the visible tracing.

PATENTEDncT 1919?:

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BACKGROUND OF THE INVENTION This invention generally relates to workpiece surface defect detectors and particularly concerns a method and apparatus for rendering visible surface defects such as fissures, cracks, and the like on billets and other semifinished products. The novel detection method and apparatus of the instant invention generally is of the type in which the workpieces or test samples are magnetized by the passage of an electric current therethrough. As is known, stray flux lines of the magnetic field are produced at the defect locations, these stray flux lines attracting a ferritic powder which is applied to the workpiece surface by means of pneumatic streams and the like.

Various techniques of this general type are known to the prior art for rendering visible fissures, cracks, and other surface defects in ferromagnetic workpieces. In one typical prior art process, an emulsion is applied to the workpiece to be tested, this emulsion penetrating the fissures and cracks. These defects are rendered visible under ultraviolet light since the emulsion normally incorporates iron-powder particles which will glow in such ultraviolet light. The fissures and cracks so recognized are then traced manually, i.e., by hand, in a dark room such that the defect locations remain visible for subsequent maintenance or repair operation after the workpiece is demagnetized. While the actual indication of the surface defects under the ultraviolet light is quite precise, the subsequent process of permanently rendering these defects visible by tracing the same detracts from the detection accuracy as this subsequent tracing process clearly is dependent upon such subjective variables as the capability and efiiciency of the human examiner. If such a testing installation is desired to be continuously operated, several examiners must alternate with one another to effect the tracing of the fissures since a human examiner cannot operate in ultraviolet light throughout the entire plant operating time.

In yet another known process, the stray flux lines of the magnetic field produced at surface defect locations on the workpiece are detected by a magnetic tape and these stray lines form signals thereon. These signals trigger the spraying of a colored stripe upon the general location of the surface defect. Yet, this process has its disadvantages in that several fissures, cracks or other defects could conceivably be disposed under a single colored stripe and thus these defects would not individually be recognizable and the subsequent maintenance or repair personnel might repair only one defect rather than all defects under a colored stripe. Additionally, this known process suffers from the disadvantage that defects such as flakes or scales disposed at the surface of the workpiece are not readily indicated.

Accordingly, all of the known processes based upon the initial magnetization of the workpiece to be tested are not capable of rendering a true fixed image or tracing of the surface defeet to the maintenance or repair personnel. This basic disadvantage of the known processes cannot be alleviated even when colored powders are used as, in each instance, only the relatively inaccurate gross indication of the defect locations is available to the subsequent repairman.

Of course, it would be of considerable economical importance if, during the actual production of workpieces such as rolling billets in a metallurgical plant, it were possible to reliably render visible the surface defects thereof and to preserve such visible indication for subsequent maintenance and repair operations. Unless such surface defects are corrected, highly deleterious effects can occur since, during further processing of a rolling billet into wire or round stock, relatively small fissures and cracks would be rolled-out further and, depending upon the diameter of the material itself, could reach considerable lengths. Accordingly, if one desires to avoid this deleterious effect, the fissures and other defects occurring in semifinished products must initially be recognized and designated in as precise a manner as possible, and such defects then must subsequently be removed.

SUMMARY OF THE INVENTION Accordingly, it is clear that a need exists in the art for a method and apparatus which, in a reliable and economical fashion, renders visible surface defects in workpieces and designates and preserves such visible indications in as precise a manner as possible for a subsequent repair or maintenance operation It is the primary object of the instant invention to provide a novel method and apparatus which satisfies this need.

This primary object as well as other objects which will become apparent as the description proceeds, are implemented by the instant invention which takes the form of both a method and an apparatus for detecting surface defects of a magnetized workpiece through the application of ferritic powder thereon and which departs from known processes of this general type in that the surface area of the workpiece which is covered with the ferritic powder is fixedly and clearly marked in an accurate manner.

In accordance with one of the inventive concepts, it has been found practical to utilize a powder for spraying which consists of agglomerated particles of both ferritic as well as nonferritic substances. Additionally, a powder having an even granular distribution can be utilized and, in this manner, an accurate image or reproduction of the defective location can be had.

In accordance with one embodiment of the novel invention, the powder agglomerate which accumulates along the surface defects to thus designate the defective locations on the workpiece surface is actually fixed by means of the application of heat in a reaction chamber. In this case, a so-called positive image or reproduction of the defective location is obtained.

In accordance with a different embodiment of the instant invention, colored marking material is applied over the surface of the workpiece after the powder agglomerates have accumulated at the defect location. The powder as well as the marking material thereon is then removed from the defect in a blowoff chamber whereas the remainder of the colored marking material over the workpiece surface is dried leaving an outline of the defect location. In this fashion, a so-called negative" indication of the defect location is obtained. Furthermore, the powder removed from the workpiece surface can be reutilized.

Thenovel apparatus for carrying out the above-described process includes a magnetizing installation for effecting magnetization of the semifinished workpiece, the installation incorporating electrodes contacting the workpiece surface.

Spraying nozzles serving to spray ferritic particles onto the workpiece surface are disposed between the electrodes and, by means of the spray nozzles, the surface of the workpiece is evenly covered. The novel apparatus further incorporates means for fixing or preserving the visible indication of the defect locations.

In accordance with one embodiment of the novel apparatus, the means for fixing or preserving the visible indication of defect locations, incorporate a colored material spraying device disposed in the magnetization path of the workpiece which serves to spray a colored substance over the surface of the workpiece upon which the ferritic powder has been applied. In this embodiment, an orientation-or position-locating device is disposed in the area of the ferritic powder spraying installation and the colored marking material spraying installation and serves to actually detect the surface error location to thus selectively activate the colored substance spraying location such that the actual permanent marking of the surface defect is effected automatically.

In a further embodiment of the novel inventive apparatus, the indicated surface error location is preserved or fixed by means of a reaction chamber disposed within the path of magnetization of the workpiece, this reaction chamber, as

described above, heating and serving to fix the ferritic powder which traces the surface defects.

So as to protect the surface defect location, the electrodes effecting the magnetization of the workpiece and particularly the electrodes contacting the discharge side of the workpiece comprise jaws disposed on an endless chain driven by a motor in synchronism with the feed speed of the workpiece itself. In th's fashion, the defective location indication is maintained in an unaltered condition for subsequent maintenance or repair operation.

BRIEF DESCRIPTION OF THE DRAWINGS The invention itself will be better understood and other advantages, objects and features thereof will become apparent when reference is given to the following detailed description of preferred inventive embodiments, such description referring to the appended sheets of drawings wherein:

FIG. 1 schematically depicts a first embodiment of an apparatus for carrying out the novel inventive method;

FIG. 2 schematically depicts an alternative embodiment of an apparatus for realizing the novel inventive process;

FIG. 3 schematically depicts a large scale plant for carrying out the process of the instant invention; and

FIG. 4 schematically depicts portions of an overall apparatus, such portions specifically illustrating a spray chamber and a magnetization path.

DETAILED DESCRIPTION OF THE PREFERRED INVENTIVE EMBODIMENTS The novel invention, both from an apparatus as well as a method standpoint, arises from the consideration that it is not sufficient to more or less inaccurately copy or trace surface defect locations initially rendered visible through the accumulation of ferritic powder particles in stray flux lines of the magnetic field and to subsequently utilize this tracing or copy for the repair of these defective locations. Rather, the novel invention is based upon the consideration that an economical repair of the surface defects is only possible if the actual accumulation of the ferritic powder particles at the surface defect locations is itself made available for the subsequent repair or maintenance operation or, alternatively, if the true, accurate reproduction of such powder accumulation is so utilized since the workpiece, when the subsequent maintenance or repair operation is effected is no longer magnetized, some means must be provided by which the true image of the ferritic powder accumulation can be obtained and fixed. Of course, the actual fixation of this image must take place during magnetization of the workpiece, i.e., during the flow of electric current therethrough.

Now, referring to the drawings and particularly to FIG. 1 thereof, a workpiece or semifinished product 1 is herein exemplified by a rod or bar having a substantially quadratic cross section. Electrodes 2 and 3 are provided for the supply of a direct electric current producing therebetween a magnetization path in which the surface defect locations are rendered visible and such defect locations are marked and preserved or fixed.

workpiece l, for example, is guided through the magnetization apparatus which, in this case, is stationarily disposed, the workpiece 1 having a given advance or feed speed. Of course, if desired, the workpiece can be stationarily disposed and the magnetization installation could be caused to travel along the length of the workpiece. In both instances, however, the novel process of the instant invention takes place in a similar fashion.

As an initial step, the workpiece is evenly sprayed with a ferromagnetic powder by means of a powder-spraying installation 4. This powder is carried upon the surface of the workpiece by means of a pneumatic air stream or current. As mentioned above, stray flux lines of the magnetic field occur at the defect locations such as surface fissures, cracks, flakes, or scales, and the ferromagnetic powder accumulates over and exactly traces the extent of the stray lines and thus of the surface defect location.

The workpiece l continues its feed movement and the accumulated powder reaches a location or position-finding device 5 serving to determine the existence of a defect location, this position-finding device 5 producing a signal actuating a colored marking or covering material spraying installation 6 which, after the workpiece has traveled further, sprays the detected defect location with a colored material substance. Thereafter, the defect location leaves the magnetization paths defined by the electrodes 2 and 3 and is demagnetized at which time the accumulated powder at the defect location at least substantially falls off. By the same token, the colored material which was subsequently sprayed over the accumulated powder by the spraying installation 6 also falls off such that a precise outline of the powder accumulation remains, which outline forms a clearly visible contrast with respect to the colored substance which remains on the remainder of the surface of the workpiece. During further advancement of the workpiece, the workpiece is exposed to an air current which serves to dry the remaining colored substance and also serves to remove any possible ferritic powder residues. It has been found that the most accurate tracing or drawing of the defect locations can be obtained when the colored coating over the workpiece surface is light and extremely thin.

The process as above described can additionally be simplified in that the utilization of a location or position-finding device 5 can be eliminated. In this event, the entire surface of the workpiece would be completely sprayed with colored material. The removal of the accumulated ferritic powder upon passing out of the magnetization path and any possible subsequent treatment such as blowing with an air current would be carried out in the same fashion in the simplified process as above discussed. Yet, a larger quantity of colored material or substance must be utilized in this instance and this necessity is balanced against the economic savings achieved by dispensing with the utilization of a location or position finding device 5 and its control apparatus.

The process as above described can be defined as a socalled negative" process, since, in this instance, it is not the powder accumulation itself which is marked and preserved or fixed but rather, it is the outline of the accumulation or the area about the actual powder accumulation on the surface of the workpiece which is fixed.

Referring now to FIG. 2, an alternative embodiment of the novel invention is disclosed, this embodiment effecting a process which can be defined as a positive" process in contrast to the negative process associated with the description of FIG. I. In FIG. 2, the workpiece again is designated by reference numeral 1 and is guided through a test installation device which, again, is assumed to be stationary. Electrodes 2 and 3 are provided which delimit the magnetization path. lnitially, the workpiece is sprayed inside the magnetization path with a ferromagnetic powder which, for example, can be applied to the workpiece surface by means of an air current. In this instance, a substance is additionally admixed to the ferromagnetic powder which substances serve to effect the adherance of the accumulated powder to the workpiece surface after the workpiece leaves the magnetization path. Subsequently, and after the workpiece has left the powder-spraying installation 4, workpiece 1 reaches a reaction chamber 7 in which the accumulated powder at the defect locations of the test piece 1 is fixed, this fixation being effected through the application of heat. The defect location is thus marked by means of the fixed accumulated powder and such marked defect location will remain after the workpiece leaves the magnetization path and the magnetization of the workpiece is removed. This marking provides an indication of the defect location which must be removed to the subsequent maintenance or repair personnel.

Both the negative as well as the positive" inventive processes as discussed can be expanded into a continuously operating plant installation. With such an installation, the

workpiece while passingthrough the testing apparatus would be sprayed with not only one, but with several ferritic powder spraying devices such that the entire circumferential surface of the workpiece is covered in a single pass. The marking and fixation of the defect location could also be carried out upon all sides of the workpiece, and, if a defect location or position finding device is provided, such device must be operative about the entire circumference of the workpiece and must supply corresponding signals for the activation of a colored material spraying installation which itself must be operative upon all workpiece sides.

Many different techniques and embodiments can be utilized forthe defect location or position-finding device 5. For example, an optical position-finding device can be provided and, in this instance, the accumulated powder would be detected by means of a photocell and titanium white colored pigments would be added to the ferromagnetic powder such that the powder creates a contrasting effect when compared with the surrounding surface of the workpiece, this contrasting effect serving to actuate the colored substance spraying installation.

Alternatively, defect position finding can take place by means of an infrared device which detects the radiation emanating from the accumulated ferritic powder which would previously have been heated so as to reach some given minimum response temperature.

As a further alternative, radioactive error or defect position finding can be utilized and in this event, radioactive material would be admixed into the ferritic powder and the radiation given off by the accumulated powder would be detected. Instead of admixing radioactive material to the ferritic powder, the powder could be exposed to X-rays and the secondary radiation emanating from the accumulated powder at the defect locations would be detected.

In each of the aboveexamples, the defect position-finding device does not have to precisely determine the defect location, that is, the precise extent of the defect. it suffices that the position-finding device merely determine the general position of the defect location such that the colored material spraying installation can act at the right location. in principle, the novel process does not even require a positionfinding device as has already been discussed and this device is utilized only for reasons of economics since, if this device were eliminated, a considerably higher consumption of coloredsubstance would result. Additionally, by utilizing the defect position-finding device, a more accurate and quicker determination of those areas having defects thereon can be obtained.

In order to be able to accurately observe the size of the defect locations and analyze the same, the ferritic powder which is utilized must possess very specific properties. In known defect testing processes based upon the magnetization of the workpiece, the powder utilized normally has an undefined granule shape and size and, thus, it is possible that defect locations which are identical may show different images even though subjected to equal magnetization. This is the case since the respective granule composition and specifically the proportion of the respective granule components are variable. As the proportion of smaller granule particles in the powder increases, the ability of the powder to reproduce smaller fissures or other defects in the workpiece surface also increases in a variable though proportional fashion. Accordingly, techniques utilizing powder having variable granule characteristics oftentimes result in false conclusions with respect to the actual nature of the defect locations even though such locations may, in fact, be directly portrayed. Such false conclusions concern determinations with respect to the type of further work to be done upon the workpiece.

In accordance with the instant invention, however, it has been determined that the above-mentioned difficulties with respect to the portrayal of surface defect locations can be avoided when a ferritic powder is utilized having an even or regular size and shape of granule, i.e., a powder having granules or particles of a size substantially disposed within a narrow range. It is therefore not necessary, yet still useful, for

the shape of the granules or particles to be similar. Good results have been obtained when utilizing particles sizes of approximately O.] to 0.4 mm. and, in this instance, the actual particles or granules are not required to consist of a single sub stance. Very favorable results have been obtained if the small ferritic particles are surrounded with a mineral substance or material which serves to glue several of such small particles together. Yet, the proportion of the mineral substance utilized must not be too great since otherwise, the magnetic properties of the powder would be reduced. When using approximately 10 percent mineral additives or substances to the powder granules, a favorable weight reduction is obtained, yet the magnetic properties remain practically unchanged from what would be the case with pure iron particles. With such agglomerated particles, bulk weights are obtained of less than 3 kg. per liter. An additional advantage to such an agglomerate construction of the particles consists in the fact that the mineral additive can be dyed whereby, during the positive" technique above-described or during the partial colored substance spraying, a portrayal or image of the defect location having sharper contrast compared with the surrounding surface of the workpiece can be obtained.

It is essential that powders which are produced in accordance with the above mentioned considerations yield powder agglomerates which are variable in size when the magnetization current changes but which are reproducible under the influence of a given strength of current for a given work piece or billet. Thus, conclusions can be drawn as to the size as well as the nature of the defect by reference to the particle accumulations which vary in width upon the fissures, flakes or other surface defects. By utilizing a powder having an even composition and further, through the choice of a suitable magnetization current, a low indication limit can be obtain below which no defect detection can be registered even when correspondingly small defect locations are present. Defect locations having a size below such limit, do not become visible and thus cannot be worked upon during the subsequent stage of maintenance and/or repair. ln practice, this lower indicator limit is in the range ofabout 0.2 to 0.3 mm. depth offissure.

By the above-described direct portrayal of powder agglomerates at the defect locations of a workpiece as well as through the use of a ferritic powder allowing one to draw conclusions with respect to both the nature and size of the defect, prerequisites are satisfied for the construction of an efficient defect testing plant or installation for testing workpieces such as rolling billets and the like, for example.

Referring now to FIG. 3, a schematic illustration of a large scale installation or plant for continuous testing of rolling billets is depicted. In such installation, which, in this instance, can be described as a longitudinal through passage installation, a workpiece 1 is guided to an infeed station 9 via infeed roller bed or table 8. At the infeed station 9, the feed or advance speed of the workpiece is somewhat reduced such that a firm contact of the front surface of successively arriving workpieces can be obtained. The actual testing station 10 incorporates the above-described components such as the magnetization path, the powder and spraying installations and devices, if desired, a position-finding device, as well as a color material spraying apparatus. A blow off device can additionally be provided in installation 10 subsequent to the magnetization path as would be required if the above-described negative process were to be undertaken in which event, the color substance which had been sprayed and deposited on the workpiece would be dried and the remainder of the ferritic powder agglomerates which had not yet completely fallen off the workpiece could be removed. A discharge roller bed or table 11 serves to guide the workpiece or billet to further transportation installations via which the billet would be led to the final repair or maintenance station. The testing station or installation can also be constructed as a transverse or crosswise through passage installation instead of as a longitudinal through passage installation as depicted. With this modification, the workpiece or billet would be fed into the testing station via a crosswise transport system. Contact with the electrodes which set up the magnetization of the workpiece would take place via the front surfaces of the workpiece. In this event, the testing station or installation could be constructed so as to be movable itself and travel along the test piece. Yet, the test station would operate in the same fashion as would be the case with a stationary installation and therefore, such test station would contain the same components. in such installations, all sides or surface of the billet can be examined for surface defects simultaneously. In the event of the provision of a test installation for testing single workpieces, or in the case of a subsequent retesting installation, it would be practical to utilize manual spray guns for the color material spray as well as for the application of the ferritic powder.

Referring now to FIG. 4, a schematic plan view of a testing station is illustrated. A spray chamber 11 incorporates a pair of rollers 12 and 13 which fonn one electrode delimiting the magnetization path. On the discharge side of the spray chamber 11, spray nozzles 14 and 15 are arranged for lateral spraying of the workpiece l. Nozzle rows are added to the spray nozzles 14 and 15 both above and below the workpiece I, so as to cover the entire surface thereof. Nozzles l4 and 15 are coupled with the pneumatically loaded bearings of the rollers l2 and 13 and serve to move with the rollers such that the distance between the nozzle outlets and the surface of the workpiece would always be the same. Reference numerals l6 and 17 serve to designate the discharge side contacting mechanism forming the second electrode delimiting the magnetization path. Contacting on the discharge side is carried out in such a manner that only a slight defacing of the test image occurs. In this respect, the contacting is effected via contact jaws 18 attached to a chain 19 driven by a nonillustrated motor in synchronism with the discharge roller bed or table.

When the apparatus is utilized to carry out the abovedescribed negative" process technique, the nonillustrated color material spraying installation would be disposed between the infeed rollers 12 and 13 and the discharge contacting elements 16 and 17.

workpiece 1, after leaving the magnetization path, passes through a blowoff device 20 which basically comprises a closed chamber having a plurality of compressed-air nozzles disposed on the inside thereof. These nozzles serve to complete the drying of the sprayed-on color material and further serve to blow off any possible ferritic powder residues, which residues are collected in the chamber and are subsequently reutilized in the process. Any powder residues which might have fallen off of the workpiece between the magnetization path and the blowoff device or installation 20 are also collected and reutilized.

The process in accordance with the instant invention contributes significantly to the improvement of the quality of rolled products. At the same time, the tracing of the various surface defect locations in semifinished products which tracing, up to now has been primarily carried out manually and which represents an exacting and tiresome workstep, is carried out mechanically herein whereby the highest possible degree of precision with respect to indication of the defect locations is achieved.

As should now be apparent, the objects initially set forth at the outset to this specification have been successfully achieved.

Accordingly, what is claimed is:

l. A process for rendering visible defects occurring at the surface of workpieces, said process including the steps of magnetizing the workpiece by passing a direct electric current therethrough at a magnetization path whereby stray flux lines of the magnetic field are caused to develop at defect locations of the workpiece; applying a ferritic powder to at least a portion of the surface of the workpiece whereby the powder is attracted by the stray flux lines and accumulates at the region of the deflect locations thereby rendering such defect locations visible; covering the surface of the workpiece which has had applied thereto the owder and at least the immediately surroundlng regions 0 the workpiece with a marking ma enal,

then removing the powder to leave a marked outline of the defect location which was previously covered by the powder, to thereby fix the position of the defect location.

2, A process as defined in claim 1, wherein the ferritic powder is sprayed onto the surface of the workpiece and comprises agglomerated particles of both ferritic and nonferritic substances.

3. A process as defined in claim 1, wherein the ferritic powder is sprayed onto the surface of the workpiece and comprises substantially even granular sizes.

4. A process as defined in claim 1, wherein the powder at a defect location is removed by directing an airstream against the workpiece, and wherein the marking material surrounding the defect location is dried by the airstream.

5. A process as defined in claim 1, further including the step of covering the entire surface area of the workpiece with the marking material.

6. An apparatus for detecting defect locations in the surface of a workpiece, said apparatus comprising: at least a pair of current carrying electrode units disposed in contact with the workpiece for passing a direct electric current therethrough at a magnetization path defined by said electrode units whereby stray flux lines of the magnetic field result at surface defect locations of the workpiece; spraying nozzle means for spraying ferritic particles evenly over the surface of the workpiece, said ferritic particles being attracted by the stray flux lines and accumulating at defect locations thereby rendering visible the surface defect locations; and means for fixing the visible indication of the surface defect locations to maintain such defect locations visible, said fixing means comprising a color marking material spraying means for spraying color marking material over the powder on the surface of the workpiece and onto at least the immediately surrounding regions of the workpiece surface, said color marking material spraying means being disposed between said pair of electrode units within the magnetization path of the workpiece.

7. An apparatus as defined in claim 6, further including a defect position-finding device disposed between said spray nozzle means and said color marking material spraying means, said defect position-finding device determining the presence ofa defect location and actuating said color marking material spraying means.

8. An apparatus as defined in claim 6, further including means for establishing a feed velocity of the workpiece with respect to said apparatus, and wherein one of said pair of electrode units at the discharge end of said apparatus comprises workpiece contact means disposed on an endless chain driven in synchronism with the feed speed of the workpiece. 

1. A process for rendering visible defects occurring at the surface of workpieces, said process including the steps of magnetizing the workpiece by passing a direct electric current therethrough at a magnetization path whereby stray flux lines of the magnetic field are caused to develop at defect locations of the workpiece; applying a ferritic powder to at least a portion of the surface of the workpiece whereby the powder is attracted by the stray flux lines and accumulates at the region of the deflect locations thereby rendering such defect locations visible; covering the surface of the workpiece which has had applied thereto the powder and at least the immediately surrounding regions of the workpiece with a marking material, then removing the powder to leave a marked outline of the defect location which was previously covered by the powder, to thereby fix the position of the defect location. CM,2Ocess as defined in claim 1, wherein the ferritic powder is sprayed onto the surface of the workpiece and comprises agglomerated particles of both ferritic and nonferritic sUbstances.
 3. A process as defined in claim 1, wherein the ferritic powder is sprayed onto the surface of the workpiece and comprises substantially even granular sizes.
 4. A process as defined in claim 1, wherein the powder at a defect location is removed by directing an airstream against the workpiece, and wherein the marking material surrounding the defect location is dried by the airstream.
 5. A process as defined in claim 1, further including the step of covering the entire surface area of the workpiece with the marking material.
 6. An apparatus for detecting defect locations in the surface of a workpiece, said apparatus comprising: at least a pair of current carrying electrode units disposed in contact with the workpiece for passing a direct electric current therethrough at a magnetization path defined by said electrode units whereby stray flux lines of the magnetic field result at surface defect locations of the workpiece; spraying nozzle means for spraying ferritic particles evenly over the surface of the workpiece, said ferritic particles being attracted by the stray flux lines and accumulating at defect locations thereby rendering visible the surface defect locations; and means for fixing the visible indication of the surface defect locations to maintain such defect locations visible, said fixing means comprising a color marking material spraying means for spraying color marking material over the powder on the surface of the workpiece and onto at least the immediately surrounding regions of the workpiece surface, said color marking material spraying means being disposed between said pair of electrode units within the magnetization path of the workpiece.
 7. An apparatus as defined in claim 6, further including a defect position-finding device disposed between said spray nozzle means and said color marking material spraying means, said defect position-finding device determining the presence of a defect location and actuating said color marking material spraying means.
 8. An apparatus as defined in claim 6, further including means for establishing a feed velocity of the workpiece with respect to said apparatus, and wherein one of said pair of electrode units at the discharge end of said apparatus comprises workpiece contact means disposed on an endless chain driven in synchronism with the feed speed of the workpiece. 